The plasma membrane is the defining feature of cells and serves as their interface with the external environment. Current models posit that it is laterally organized in domains of distinct protein and lipid composition that are important for the efficient orchestration of key reactions of transport and communication, for example involved in endocytosis. However, despite tremendous advances in the comprehension of membrane processes and structure, physiological functions of plasma membrane organization are not yet clear. To overcome limitations of complex models used for investigation, we will focus on studying plasma membrane organization in yeast. This model system has prominent protein and lipid segregation in the plasma membrane and is amenable to molecular, biochemical, genetic and systems approaches. In this project, we will capitalize on our discovery of eisosomes, large protein complexes underlying the plasma membrane, as principal organizers plasma membrane domains in yeast. We aim to define the molecular mechanisms and cellular functions of plasma membrane organization. We hypothesize that eisosomes assemble into a stable membrane scaffold that plays fundamental roles in phosphoinositide cell biology and endocytosis. To test this model, we will use directed biochemical, structural biology, cell biology experiments combined with unbiased analytical tools, including state-of-the-art proteomics and systematic genetics. By tackling these central questions on plasma membrane biology and elucidating the function of membrane organization by eisosomes, we will address a fundamental cell biology problem. Salient features of biological systems, including the structure of eisosome proteins are most often evolutionary conserved. Our findings will therefore likely have a broad impact on membrane research. They might also have therapeutic implications for a wide range of human pathologies where plasma membrane organization is implicated.